2, U.S. Army Research Laboratory, Adelphi, Maryland, United States
While there have been recent advances in solid ion conductors exhibiting conductivities comparable to liquid electrolytes, how to best capitalize on these materials discoveries to enable new energy storage technology is currently not known. Of particular interest is the integration solid-state electrolyte into solid-state batteries; however, numerous questions remain. What are the design rules? How are solid-solid interfaces formed? How does charge transfer occur at interfaces?
Essentially, there are two interfaces of interest in solid-state batteries, distinguished by the electrode type; the alkali metal electrode and the ceramic electrode. Several key scientific challenges related to these two interfaces must be addressed to mature solid-state batteries. This discussion is centered on the alkali metal/solid electrolyte interface. The solid electrolyte based on garnet-type oxide, of nominal composition Li7La3Zr2O12 (LLZO), is used as a model system that simultaneously exhibits fast-ion conductivity and stability against metallic Li.
To date, there are few examples of bulk-scale Li-ion conducting solid electrolytes that are stable at the Li/Li+ redox potential. This paper elucidates stability and kinetics as a function of LLZO composition and surface chemistry. EIS, TEM, and electron spin resonance analysis describes the subtle interaction at the interface between Li metal and LLZO. Simple approaches to achieving negligible Li-LLZO interface resistance (~1 Ohm.cm-2) with no coatings is described. Based on this mechanism, a simple procedure for removing these surface layers is demonstrated, resulting in a dramatic increase in Li wetting and the elimination of nearly all interfacial resistance. Combined, the demonstrated stability and low interfacial resistance suggests a pathway to achieving viable high energy density solid-state batteries enabled by the LLZO-based solid electrolyte.